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  • richardmitnick 5:11 pm on January 29, 2021 Permalink | Reply
    Tags: "Discovery machines", , ADONE ( 1969-1993 ) at Frascati (IT), Antiproton Accumulator CERN, Axial Field Spectrometer (AFS) CERN, , Bevatron at Lawrence Berkeley National Laboratory, Brookhaven’s Alternating Gradient Synchrotron, CERN (CH) Courier, CERN Intersecting Storage Rings (ISR), , , CERN’s Proton Synchrotron, From CERN (CH) Courier, Gargamelle heavy-liquid bubble chamber CERN, , Initial Cooling Experiment (ICE) 1977–1978 CERN, Intersecting Storage Rings (ISR) CERN, New era CERN’s Intersecting Storage Rings in 1974, , , , Proton Synchrotron at Serpukov near Moscow, Soviet VEP-1; VEPP-2 and VEPP-2M between 1963 and 1974, SPEAR at SLAC, Split Field Magnet facility 1977-2021 CERN., Super collider CERN’s SppS in 1983, Superconducting Super Collider to be built in Texas killed off in 1993 by the US idiot Congress, The Cosmotron at Brookhaven National Laboratory, The Super Proton Synchrotron (SPS) CERN, The Tevatron at Fermilab 1983-2011, UA1 and UA2 at CERN,   

    From CERN (CH) Courier: “Discovery machines” 

    From CERN (CH) Courier


    27 January 2021
    Lyn Evans, (former LHC project director), Imperial College London(UK)
    Peter Jenni, former ATLAS spokesperson), Albert Ludwig University of Freiburg (DE) and CERN.

    New era CERN’s Intersecting Storage Rings in 1974. Credit: CERN-PHOTO-7408061

    The ability to collide high-energy beams of hadrons under controlled conditions transformed the field of particle physics. Until the late 1960s, the high-energy frontier was dominated by the great proton synchrotrons. The Cosmotron at Brookhaven National Laboratory and the Bevatron at Lawrence Berkeley National Laboratory were soon followed by CERN’s Proton Synchrotron and Brookhaven’s Alternating Gradient Synchrotron, and later by the Proton Synchrotron at Serpukov near Moscow [image N/A].

    BNL Cosmotron.

    LBNL Bevatron.

    CERN Proton Synchrotron

    BNL Alternating Gradient Synchrotron (1960-present)

    In these machines protons were directed to internal or external targets in which secondary particles were produced.

    The kinematical inefficiency of this process, whereby the centre-of-mass energy only increases as the square root of the beam energy, was recognised from the outset. In 1943, Norwegian engineer Rolf Widerøe proposed the idea of colliding beams, keeping the centre of mass at rest in order to exploit the full energy for the production of new particles. One of the main problems was to get colliding beam intensities high enough for a useful event rate to be achieved. In the 1950s the prolific group at the University of Wisconsin Midwestern Universities Research Association (MURA), led by Donald Kerst, worked on the problem of “stacking” particles, whereby successive pulses from an injector synchrotron are superposed to increase the beam intensity. They mainly concentrated on protons, where Liouville’s theorem (which states that for a continuous fluid under the action of conservative forces the density of phase space cannot be increased) was thought to apply. Only much later, ways to beat Liouville and to increase the beam density were found. At the 1956 International Accelerator Conference at CERN, Kerst made the first proposal to use stacking to produce colliding beams (not yet storage rings) of sufficient intensity.

    Super collider CERN’s SppS in 1983. Credit: CERN-AC-7604110.

    At that same conference, Gerry O’Neill from Princeton presented a paper proposing that colliding electron beams could be achieved in storage rings by making use of the natural damping of particle amplitudes by synchrotron-radiation emission. A design for the 500 MeV Princeton–Stanford colliding beam experiment was published in 1958 and construction started that same year. At the same time, the Budker Institute for Nuclear Research in Novosibirsk started work on VEP-1, a pair of rings designed to collide electrons at 140 MeV.

    Between 1963 and 1974 Soviet physicists named VEP-1, VEPP-2 and VEPP-2M three different colliders they built it. However, by the 1980s, the Soviets’ work on collider began to slow down. While other countries that keep pace with new technologies are building next-generation colliders, Soviet scientists In the technology of the 1970s they were stuck.

    Then, in March 1960, Bruno Touschek gave a seminar at Laboratori Nazionali di Frascati in Italy where he first proposed a single-ring, 0.6 m-circumference 250 MeV electron–positron collider. “AdA” produced the first stored electron and positron beams less than one year later – a far cry from the time it takes today’s machines to go from conception to operation! From these trailblazers evolved the production machines, beginning with ADONE at Frascati and SPEAR at SLAC. However, it was always clear that the gift of synchrotron-radiation damping would become a hindrance to achieving very high energy collisions in a circular electron–positron collider because the power radiated increases as the fourth power of the beam energy and the inverse fourth power of mass, so is negligible for protons compared with electrons.

    ADONE ( 1969-1993 ) at Frascati (IT)

    SPEAR at SLAC.

    A step into the unknown

    Meanwhile, in the early 1960s, discussion raged at CERN about the next best step for particle physics. Opinion was sharply divided between two camps, one pushing a very high-energy proton synchrotron for fixed-target physics and the other using the technique proposed at MURA to build an innovative colliding beam proton machine with about the same centre-of-mass energy as a conventional proton synchrotron of much larger dimensions. In order to resolve the conflict, in February 1964, 50 physicists from among Europe’s best met at CERN. From that meeting emerged a new committee, the European Committee for Future Accelerators, under the chairmanship of one of CERN’s founding fathers, Edoardo Amaldi. After about two years of deliberation, consensus was formed. The storage ring gained most support, although a high-energy proton synchrotron, the Super Proton Synchrotron (SPS), was built some years later and would go on to play an essential role in the development of hadron storage rings.

    The Super Proton Synchrotron (SPS), CERN’s second-largest accelerator.

    On 15 December 1965, with the strong support of Amaldi, the CERN Council unanimously approved the construction of the Intersecting Storage Rings (ISR), launching the era of hadron colliders.

    Intersecting Storage Rings (ISR). Credit: CERN.

    First collisions

    Construction of the ISR began in 1966 and first collisions were observed on 27 January 1971. The machine, which needed to store beams for many hours without the help of synchrotron-radiation damping to combat inevitable magnetic field errors and instabilities, pushed the boundaries in accelerator science on all fronts. Several respected scientists doubted that it would ever work. In fact, the ISR worked beautifully, exceeding its design luminosity by an order of magnitude and providing an essential step in the development of the next generation of hadron colliders. A key element was the performance of its ultra-high-vacuum system, which was a source of continuous improvement throughout the 13 year-long lifetime of the machine.

    For the experimentalists, the ISR’s collisions (which reached an energy of 63 GeV) opened an exciting adventure at the energy frontier. But they were also learning what kind of detectors to build to fully exploit the potential of the machine – a task made harder by the lack of clear physics benchmarks known at the time in the ISR energy regime. The concept of general-purpose instruments built by large collaborations, as we know them today, was not in the culture of the time. Instead, many small collaborations built experiments with relatively short lifecycles, which constituted a fruitful learning ground for what was to come at the next generation of hadron colliders.

    There was initially a broad belief that physics action would be in the forward directions at a hadron collider. This led to the Split Field Magnet facility as one of the first detectors at the ISR, providing a high magnetic field in the forward directions but a negligible one at large angle with respect to the colliding beams (the nowadays so-important transverse direction).

    Split Field Magnet facility. © 1977-2021 CERN.

    It was with subsequent detectors featuring transverse spectrometer arms over limited solid angles that physicists observed a large excess of high transverse momentum particles above low-energy extrapolations. With these first observations of point-like parton scattering, the ISR made a fundamental contribution to strong-interaction physics. Solid angles were too limited initially, and single-particle triggers too biased, to fully appreciate the hadronic jet structure. That feat required third-generation detectors, notably the Axial Field Spectrometer (AFS) at the end of the ISR era, offering full azimuthal central calorimeter coverage.

    Axial Field Spectrometer (AFS). Credit: CERN.

    The experiment provided evidence for the back-to-back two-jet structure of hard parton scattering.

    TeV frontier The Tevatron at Fermilab in 2011. Credit: Fermilab.

    For the detector builders, the original AFS concept was interesting as it provided an unobstructed phi-symmetric magnetic field in the centre of the detector, however, at the price of massive Helmholtz coil pole tips obscuring the forward directions. Indeed, the ISR enabled the development of many original experimental ideas. A very important one was the measurement of the total cross section using very forward detectors in close proximity to the beam. These “Roman Pots”, named for their inventors, made their appearance in all later hadron colliders, confirming the rising total pp cross section with energy.

    It is easy to say after the fact, still with regrets, that with an earlier availability of more complete and selective (with electron-trigger capability) second- and third-generation experiments at the ISR, CERN would not have been left as a spectator during the famous November revolution of 1974 with the J/ψ discoveries at Brookhaven and SLAC. These, and the ϒ resonances discovered at Fermilab three years later, were clearly observed in the later-generation ISR experiments.

    SPS opens new era

    However, events were unfolding at CERN that would pave the way to the completion of the Standard Model.

    Standard Model of Particle Physics (LATHAM BOYLE AND MARDUS OF WIKIMEDIA COMMONS).

    At the ISR in 1972, the phenomenon of Schottky noise (density fluctuations due to the granular nature of the beam in a storage ring) was first observed. It was this very same noise that Simon van der Meer speculated in a paper a few years earlier could be used for what he called “stochastic cooling” of a proton beam, beating Liouville’s theorem by the fact that a beam of particles is not a continuous fluid. Although it is unrealistic to detect the motion of individual particles and damp them to the nominal orbit, van der Meer showed that by correcting the mean transverse motion of a sample of particles continuously, and as long as the statistical nature of the Schottky signal was continuously regenerated, it would be theoretically possible to reduce the beam size and increase its density. With the bandwidth of electronics available at the time, van der Meer concluded that the cooling time would be too long to be of practical importance. But the challenge was taken up by Wolfgang Schnell, who built a state-of-the-art feedback system that demonstrated stochastic cooling of a proton beam for the first time. This would open the door to the idea of stacking and cooling of antiprotons, which later led to the SPS being converted into a proton–antiproton collider.

    Big beast The Large Hadron Collider in 2018. Credit: CERN-PHOTO-201802-030.

    Another important step towards the next generation of hadron colliders occurred in 1973 when the collaboration working on the Gargamelle heavy-liquid bubble chamber published two papers revealing the first evidence for weak neutral currents.

    The Gargamelle heavy-liquid bubble chamber during its installation at the Proton Synchrotron in September 1970. Credit: CERN.

    These were important observations in support of the unified theory of electromagnetic and weak interactions, for which Sheldon Glashow, Abdus Salam and Steven Weinberg were to receive the Nobel Prize in Physics in 1979. The electroweak theory predicted the existence and approximate masses of two vector bosons, the W and the Z, which were too high to be produced in any existing machine. However, Carlo Rubbia and collaborators proposed that, if the SPS could be converted into a collider with protons and antiprotons circulating in opposite directions, there would be enough energy to create them.

    To achieve this the SPS would need to be converted into a storage ring like the ISR, but this time the beam would need to be kept “bunched” with the radio-frequency (RF) system working continuously to achieve a high enough luminosity (unlike the ISR where the beams were allowed to de-bunch all around the ring). The challenges here were two-fold. Noise in the RF system causes particles to diffuse rapidly from the bunch. This was solved by a dedicated feedback system. It was also predicted that the beam–beam interaction would limit the performance of a bunched-beam machine with no synchrotron-radiation damping due to the strongly nonlinear interactions between a particle in one beam with the global electromagnetic field in the other beam.

    A much bigger challenge was to build an accumulator ring in which antiprotons could be stored and cooled by stochastic cooling until a sufficient intensity of antiprotons would be available to transfer into the SPS, accelerate to around 300 GeV and collide with protons. This was done in two stages. First a proof-of-principle was needed to show that the ideas developed at the ISR transferred to a dedicated accumulator ring specially designed for stochastic cooling. This ring was called the Initial Cooling Experiment (ICE), and operated at CERN in 1977–1978.

    Initial Cooling Experiment (ICE) 1977–1978. Credit: CERN.

    In ICE transverse cooling was applied to reduce the beam size and a new technique for reducing the momentum spread in the beam was developed. The experiment proved to be a big success and the theory of stochastic cooling was refined to a point where a real accumulator ring (the Antiproton Accumulator) could be designed to accumulate and store antiprotons produced at 3.5 GeV by the proton beam from the 26 GeV Proton Synchrotron.

    Antiproton Accumulator. Credit: CERN.

    First collisions of protons and antiprotons at 270 GeV were observed on the night of 10 July 1981, signalling the start of a new era in colliding beam physics.

    First steps The R702 experiment at the ISR in 1977. Credit: CERN-PHOTO-7708541X.

    A clear physics goal, namely the discovery of the W and Z intermediate vector bosons, drove the concepts for the two main SppS experiments UA1 and UA2 (in addition to a few smaller, specialised experiments).

    UA1. Credit: CERN.

    US2. Credit: CERN.

    It was no coincidence that the leaders of both collaborations were pioneers of ISR experiments, and many lessons from the ISR were taken on board. UA1 pioneered the concept of a hermetic detector that covered as much as possible the full solid angle around the interaction region with calorimetry and tracking. This allows measurements of the missing transverse energy/momentum, signalling the escaping neutrino in the leptonic W decays. Both electrons and muons were measured, with tracking in a state-of-the-art drift chamber that provided bubble-chamber-like pictures of the interactions. The magnetic field was provided by a dipole-magnet configuration, an approach not favoured in later generation experiments because of its inherent lack of azimuthal symmetry. UA2 featured a (at the time) highly segmented electromagnetic and hadronic calorimeter in the central part (down to 40 degrees with respect to the beam axis), with 240 cells pointing to the interaction region. But it had no muon detection, and in its initial phase only limited electromagnetic coverage in the forward regions. There was no magnetic field except for the forward cones with toroids to probe the W polarisation.

    In 1983 the SppS experiments made history with the direct discoveries of the W and Z. Many other results were obtained, including the first evidence of neutral B-meson particle–antiparticle mixing at UA1 thanks to its tracking and muon detection. The calorimetry of UA2 provided immediate unambiguous evidence for a two-jet structure in events with large transverse energy. Both UA1 and UA2 pushed QCD studies far ahead. The lack of hermeticity in UA2’s forward regions motivated a major upgrade (UA2′) for the second phase of the collider, complementing the central part with new fully hermetic calorimetry (both electromagnetic and hadronic), and also inserting a new tracking cylinder employing novel technologies (fibre tracking and silicon pad detectors). This enabled the experiment to improve searches for top quarks and supersymmetric particles, as well as making almost background-free first precision measurements of the W mass.

    Meanwhile in America

    At the time the SppS was driving new studies at CERN, the first large superconducting synchrotron (the Tevatron, with a design energy close to 1 TeV) was under construction at Fermilab.

    FNAL/Tevatron map

    Tevatron Accelerator


    In view of the success of the stochastic cooling experiments, there was a strong lobby at the time to halt the construction of the Tevatron and to divert effort instead to emulate the SPS as a proton–antiproton collider using the Fermilab Main Ring. Wisely this proposal was rejected and construction of the Tevatron continued. It came into operation as a fixed-target synchrotron in 1984. Two years later it was also converted into a proton–antiproton collider and operated at the high-energy frontier until its closure in September 2011.

    A huge step was made with the detector concepts for the Tevatron experiments, in terms of addressed physics signatures, sophistication and granularity of the detector components. This opened new and continuously evolving avenues in analysis methods at hadron colliders. Already the initial CDF and DØ detectors for Run I (which lasted until 1996) were designed with cylindrical concepts, characteristic of what we now call general-purpose collider experiments, albeit DØ still without a central magnetic field in contrast to CDF’s 1.4 T solenoid.

    FNAL/Tevatron CDF detector

    FNAL/Tevatron DZero detector

    In 1995 the experiments delivered the first Tevatron highlight: the discovery of the top quark. Both detectors underwent major upgrades for Run II (2001–2011) – a theme now seen for the LHC experiments – which had a great impact on the Tevatron’s physics results. CDF was equipped with a new tracker, a silicon vertex detector, new forward calorimeters and muon detectors, while DØ added a 1.9 T central solenoid, vertexing and fibre tracking, and new forward muon detectors. Alongside the instrumentation was a breath-taking evolution in real-time event selection (triggering) and data acquisition to keep up with the increasing luminosity of the collider.

    The physics harvest of the Tevatron experiments during Run II was impressive, including a wealth of QCD measurements and major inroads in top-quark physics, heavy-flavour physics and searches for phenomena beyond the Standard Model. Still standing strong are its precision measurements of the W and top masses and of the electroweak mixing angle sin2θW. The story ended in around 2012 with a glimpse of the Higgs boson in associated production with a vector boson. The CDF and DØ experience influenced the LHC era in many ways: for example they were able to extract the very rare single-top production cross-section with sophisticated multivariate algorithms, and they demonstrated the power of combining mature single-experiment measurements in common analyses to achieve ultimate precision and sensitivity.

    For the machine builders, the pioneering role of the Tevatron as the first large superconducting machine was also essential for further progress. Two other machines – the Relativistic Heavy Ion Collider at Brookhaven and the electron–proton collider HERA at DESY – derived directly from the experience of building the Tevatron.

    BNL RHIC Campus.


    BNL/RHIC Star Detector

    BNL/RHIC Phenix.

    Lessons learned from that machine and from the SppS were also integrated into the design of the most powerful hadron collider yet built: the LHC [below].

    The Large Hadron Collider

    The LHC had a difficult birth. Although the idea of a large proton–proton collider at CERN had been around since at least 1977, the approval of the Superconducting Super Collider (SSC) in the US in 1987 put the whole project into doubt. The SSC, with a centre-of-mass energy of 40 TeV, was almost three times more powerful than what could ever be built using the existing infrastructure at CERN. It was only the resilience and conviction of Carlo Rubbia, who shared the 1984 Nobel Prize in Physics with van der Meer for the project leading to the discovery of the W and Z bosons, that kept the project alive. Rubbia, who became Director-General of CERN in 1989, argued that, in spite of its lower energy, the LHC could be competitive with the SSC by having a luminosity an order of magnitude higher, and at a fraction of the cost. He also argued that the LHC would be more versatile: as well as colliding protons, it would be able to accelerate heavy ions to record energies at little extra cost.

    The SSC was eventually cancelled in 1993. This made the case for the LHC even stronger, but the financial climate in Europe at the time was not conducive to the approval of a large project. For example, CERN’s largest contributor, Germany, was struggling with the cost of reunification and many other countries were getting to grips with the introduction of the single European currency. In December 1993 a plan was presented to the CERN Council to build the machine over a 10-year period by reducing the other experimental programmes at CERN to the absolute minimum, with the exception of the full exploitation of the flagship Large Electron Positron (LEP) collider. Although the plan was generally well received, it became clear that Germany and the UK were unlikely to agree to the budget increase required. On the positive side, after the demise of the SSC, a US panel on the future of particle physics recommended that “the government should declare its intentions to join other nations in constructing the LHC”. Positive signals were also being received from India, Japan and Russia.

    In June 1994 the proposal to build the LHC was made once more. However, approval was blocked by Germany and the UK, which demanded substantial additional contributions from the two host states, France and Switzerland. This forced CERN to propose a “missing magnet” machine where only two thirds of the dipole magnets would be installed in a first stage, allowing operation at reduced energy for a number of years. Although costing more in the long run, the plan would save some 300 million Swiss Francs in the first phase. This proposal was put to Council in December 1994 by the new Director-General Christopher Llewellyn Smith and, after a round of intense discussions, the project was finally approved for two-stage construction, to be reviewed in 1997 after non-Member States had made known their contributions. The first country to do so was Japan in 1995, followed by India, Russia and Canada the next year. A final sting in the tail came in June 1996 when Germany unilaterally announced that it intended to reduce its CERN subscription by between 8% and 9%, prompting the UK to demand a similar reduction and forcing CERN to take out loans. At the same time, the two-stage plan was dropped and, after a shaky start, the construction of the full LHC was given the green light.

    The fact that the LHC was to be built at CERN, making full use of the existing infrastructure to reduce cost, imposed a number of strong constraints. The first was the 27 km-circumference of the LEP tunnel in which the machine was to be housed. For the LHC to achieve its design energy of 7 TeV per beam, its bending magnets would need to operate at a field of 8.3 T, about 60% higher than ever achieved in previous machines. This could only be done using affordable superconducting material by reducing the temperature of the liquid-helium coolant from its normal boiling point of 4.2 K to 1.9 K – where helium exists in a macroscopic quantum state with the loss of viscosity and a very large thermal conductivity. A second major constraint was the small (3.8 m) tunnel diameter, which made it impossible to house two independent rings like the ISR. Instead, a novel and elegant magnet design, first proposed by Bob Palmer at Brookhaven, with the two rings separated by only 19 cm in a common yoke and cryostat was developed. This also considerably reduced the cost.

    At precisely 09:30 on 10 September 2008, almost 15 years after the project’s approval, the first beam was injected into the LHC, amid global media attention. In the days that followed good progress was made until disaster struck: during a ramp to full energy, one of the 10,000 superconducting joints between the magnets failed, causing extensive damage from which it took more than a year to recover. Following repairs and consolidation, on 29 November 2009 beam was once more circulating and full commissioning and operation could start. Rapid progress in ramping up the luminosity followed, and the LHC physics programme, at an initial energy of 3.5 TeV per beam, began in earnest in March 2010.

    LHC experiments

    Yet a whole other level of sophistication was realised by the LHC detectors compared to those at previous colliders. The priority benchmark for the designs of the general-purpose detectors ATLAS [below] and CMS [below] was to unambiguously discover (or rule out) the Standard Model Higgs boson for all possible masses up to 1 TeV, which demanded the ability to measure a variety of final states. The challenges for the Higgs search also guaranteed the detectors’ potential for all kinds of searches for physics beyond the Standard Model, which was the other driving physics motivation at the energy frontier. These two very ambitious LHC detector designs integrated all the lessons learned from the experiments at the three predecessor machines, as well as further technology advances in other large experiments, most notably at HERA and LEP.

    H1 detector at DESY HERA ring.

    CERN LEP Collider.

    Just a few simple numbers illustrate the giant leap from the Tevatron to the LHC detectors. CDF and DØ, in their upgraded versions operating at a luminosity of up to 4 × 1032 cm–2s–1, typically had around a million channels and a triggered event rate of 100 Hz, with event sizes of 500 kB. The collaborations were each about 600 strong. By contrast, ATLAS and CMS operated during LHC Run 2 at a luminosity of 2 × 1034 cm–2s–1 with typically 100 million readout channels, and an event rate and size of 500 Hz and 1500 kB. Their publications have close to 3000 authors.

    For many major LHC-detector components, complementary technologies were selected. This is most visible for the superconducting magnet systems, with an elegant and unique large 4 T solenoid in CMS serving both the muon and inner tracking measurements, and an air-core toroid system for the muon spectrometer in ATLAS together with a 2 T solenoid around the inner tracking cylinder. These choices drove the layout of the active detector components, for instance the electromagnetic calorimetry. Here again, different technologies were implemented: a novel-configuration liquid-argon sampling calorimeter for ATLAS and lead-tungstate crystals for CMS.

    From the outset, the LHC was conceived as a highly versatile collider facility, not only for the exploration of high transverse-momentum physics. With its huge production of b and c quarks, it offered the possibility of a very fruitful programme in flavour physics, exploited with great success by the purposely designed LHCb experiment [below]. Furthermore, in special runs the LHC provides heavy-ion collisions for studies of the quark–gluon plasma – the field of action for the ALICE experiment below].

    As the general-purpose experiments learned from the history of experiments in their field, the concepts of both LHCb and ALICE also evolved from a previous generation of experiments in their fields, which would be interesting to trace back. One remark is due: the designs of all four main detectors at the LHC have turned out to be so flexible that there are no strict boundaries between these three physics fields for them. All of them have learned to use features of their instruments to contribute at least in part to the full physics spectrum offered by the LHC, of which the highlight so far was the July 2012 announcement of the discovery of the Higgs boson by the ATLAS and CMS collaborations.

    Peter Higgs

    CERN CMS Higgs Event May 27, 2012.

    CERN ATLAS Higgs Event
    June 12, 2012.

    The following year the collaborations were named in the citation for the 2013 Nobel Prize in Physics awarded to François Englert and Peter Higgs.

    CMS undergoing upgrades in 2019. Credit: CERN.

    Since then, the LHC has exceeded its design luminosity by a factor of two and delivered an integrated luminosity of almost 200 fb–1 in proton–proton collisions, while its beam energy was increased to 6.5 TeV in 2015. The machine has also delivered heavy ion (lead–lead) and even lead–proton collisions. But the LHC still has a long way to go before its estimated end of operations in the mid-to-late 2030s. To this end, the machine was shut down in November 2018 for a major upgrade of the whole of the CERN injector complex as well as the detectors to prepare for operation at high luminosities, ultimately up to a “levelled” luminosity of 7 × 1034 cm–2s–1. The High Luminosity LHC (HL-LHC) upgrade is pushing the boundaries of superconducting magnet technology to the limit, particularly around the experiments where the present focusing elements will be replaced by new magnets built from high-performance Nb3Sn superconductor. The eventual objective is to accumulate 3000 fb–1 of integrated luminosity.

    In parallel, the LHC-experiment collaborations are preparing and implementing major upgrades to their detectors using novel state-of-art technologies and revolutionary approaches to data collection to exploit the tenfold data volume promised by the HL-LHC. Hadron-collider detector concepts have come a long way in sophistication over the past 50 years. However, behind the scenes are other factors paramount to their success. These include an equally spectacular evolution in data-flow architectures, software and the computing approaches, and analysis methods – all of which have been driven into new territories by the extraordinary needs for dealing with rare events within the huge backgrounds of ordinary collisions at hadron colliders. Worthy of particular mention in the success of all LHC physics results is the Worldwide LHC Computing Grid.

    MonALISA LHC Computing GridMap monalisa.caltech.edu/ml/_client.beta

    This journey is now poised to continue, as we look ahead towards how a general-purpose detector at a future 100 TeV hadron collider might look like.

    Beyond the LHC

    Although the LHC has at least 15 years of operations ahead of it, the question now arises, as it did in 1964: what is the next step for the field? The CERN Council has recently approved the recommendations of the 2020 update of the European strategy for particle physics, which includes, among other things, a thorough study of a very high-energy hadron collider to succeed the LHC.

    CERN FCC Future Circular Collider details of proposed 100km-diameter successor to LHC.

    A technical and financial feasibility study for a 100 km circular collider at CERN with a collision energy of at least 100 TeV is now under way. While a decision to proceed with such a facility is to come later this decade, one thing is certain: lessons learned from 50 years of experience with hadron colliders and their detectors will be crucial to the success of our next step into the unknown.

    A possible future elsewhere

    China Circular Electron Positron Collider (CEPC) map. It would be housed in a hundred-kilometer- (62-mile-) round tunnel at one of three potential sites. The documents work under the assumption that the collider will be located near Qinhuangdao City around 200 miles east of Beijing.

    See the full article here .

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  • richardmitnick 10:17 pm on January 14, 2021 Permalink | Reply
    Tags: "HL-LHC magnets enter production in the US", , , , CERN (CH) Courier, , , , , , , , US LHC Accelerator Research Program (LARP)   

    From CERN (CH) Courier: “HL-LHC magnets enter production in the US” 

    From CERN (CH) Courier

    13 January 2021
    Matthew Chalmers editor.

    Next generation BNL technicians Ray Ceruti, Frank Teich, Pete Galioto, Pat Doutney and Dan Sullivan with the second US quadrupole magnet for the HL-LHC to have reached design performance. Credit: BNL.

    The significant increase in luminosity targeted by the high-luminosity LHC (HL-LHC) demands large-aperture quadrupole magnets that are able to focus the proton beams more tightly as they collide. A total of 24 such magnets are to be installed on either side of the ATLAS and CMS experiments [both below] in time for HL-LHC operations in 2027, marking the first time niobium-tin (Nb3Sn) magnet technology is used in an accelerator.

    Nb3Sn is a superconducting material with a critical magnetic field that far exceeds that of the niobium-titanium presently used in the LHC magnets, but once formed it becomes brittle and strain-sensitive, which makes it much more challenging to process and use.

    The milestone signals the end of the prototyping phase for the HL-LHC quadrupoles.

    Following the first successful test of a US-built HL-LHC quadrupole magnet at Brookhaven National Laboratory (BNL) in January last year—attaining a conductor peak field of 11.4 T and exceeding the required integrated gradient of 556 T in a 150 mm-aperture bore—a second quadrupole magnet has now been tested at BNL at nominal performance. Since the US-built quadrupole magnets must be connected in pairs before they can constitute fully operational accelerator magnets, the milestone signals the end of the prototyping phase for the HL-LHC quadrupoles, explains Giorgio Apollinari of Fermilab, who is head of the US Accelerator Upgrade Projects (AUP). “The primary importance is that we have entered the ‘production’ period that will make installation viable in early 2025. It also means we have satisfied the requirements from our funding agency and now the US Department of Energy has authorised the full construction for the US contribution to HL-LHC.”

    Joint venture

    The design and production of the HL-LHC quadrupole magnets are the result of a joint venture between CERN, BNL, Fermilab and Lawrence Berkeley National Laboratory, preceded by the 15 year-long US LHC Accelerator Research Program (LARP).

    The US labs are to provide a total of ten 9 m-long helium-tight vessels (eight for installation and two as spares) for the HL-LHC, each containing two 4.2 m-long magnets. CERN is also producing ten 9 m-long vessels, each containing a 7.5 m-long magnet. The six magnets to be placed on each side of ATLAS and CMS – four from the US and two from CERN – will be powered in series on the same electrical circuit.

    The synergy between CERN and the US laboratories allowed us to considerably reduce the risks.

    “The synergy between CERN and the US laboratories allowed us to considerably reduce the risks, have a faster schedule and a better optimisation of resources,” says Ezio Todesco of CERN’s superconductors and cryostats group. The quadrupole magnet programme at CERN is also making significant progress, he adds, with a short-model quadrupole having recently reached a record 13.4 T peak field in the coil, which is 2 T more than the project requirements. “The full series of magnets, sharing the same design and built on three sites, will also give very relevant information about the viability of future hadron colliders, which are expected to rely on massive, industrial production of Nb3Sn magnets with fields up to 16 T.”

    Since the second US quadrupole magnet was tested in October, the AUP teams have completed the assembly of a third magnet and are close to completing the assembly of a fourth. Next, the first two magnets will be assembled in a single cold mass before being tested in a horizontal configuration and then shipped to CERN in time for the “string test” planned in 2023.

    “In all activities at the forefront of technology, like in the case for these focusing Nb3Sn quadrupoles, the major challenge is probably the transition from an ‘R&D mentality’, where minor improvements can be a daily business, to a ‘production mentality’, where there is a need to build to specific procedures and criteria, with all deviations being formally treated and corrected or addressed,” says Apollinari. “And let’s not forget that the success of this second magnet test came with a pandemic raging across the world.”

    See the full article here .

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  • richardmitnick 5:11 pm on November 11, 2020 Permalink | Reply
    Tags: "TESLA’s high-gradient march", Accelerating gradients above 40 MV/m are now attainable with niobium., , CERN (CH) Courier, , TESLA Technology Collaboration-DESY (DE)   

    From CERN (CH) Courier: “TESLA’s high-gradient march” 

    From CERN (CH) Courier

    10 November 2020
    Eiji Kako, KEK (JP)
    Paolo Pierini ESS (SE)
    Charles E Reece JLab (US)

    Established 30 years ago with a linear electron–positron collider in mind, the TESLA Technology Collaboration-DESY (DE) has played a major role in the development of superconducting radio-frequency cavities and related technologies for a wide variety of applications.

    Standing tall Superconducting radio-frequency cavities at DESY (DE). Credit: D Noelle/DESY.

    Energetic beams of charged particles are essential for high-energy physics research, as well as for studies of nuclear structure and dynamics, and deciphering complex molecular structures. In principle, generating such beams is simple: provide an electric field for acceleration and a magnetic field for bending particle trajectories. In practice, however, the task becomes increasingly challenging as the desired particle energy goes up. Very high electric fields are required to attain the highest energy beams within practical real-estate constraints.

    The most efficient way to generate the very high electric fields in a vacuum environment required to transport a beam is to build up a resonant excitation of radio waves inside a metallic cavity. There is something of an art to shaping such cavities to “get the best bang for the buck” for a particular application. The radio-frequency (RF) fields are inherently time-varying, and bunches of charged particles need to arrive with the right timing if they are to see only forward-accelerating electric fields. Desirable very high resonant electric fields (e.g. 5–40 MV/m) require the existence of very high currents in the cavity walls. These currents are simply not sustainable for long durations using even the best normal-conducting materials, as they would melt from resistive heating.

    Superconducting materials, on the other hand, can support sustainable high-accelerating gradients with an affordable electricity bill. Early pioneering work demonstrating the first beam-acceleration using superconducting radio-frequency (SRF) cavities took place in the late 1960s and early 1970s at Stanford, Caltech, the University of Wuppertal (DE) and Karlsruhe Institute of Technology (DE). The potential for real utility was clear, but techniques and material refinements were needed. Several individual laboratories began to take up the challenge for their own research needs. Solutions were developed for electron acceleration at CESR (FR), HERA (DE), TRISTAN (JP), LEP II (CH) and CEBAF (US), while heavy-ion SRF acceleration solutions were developed at Stony Brook (US), ATLAS (CH), ALPI and others. The community of SRF accelerator physicists was small but the lessons learned were consistently shared and documented. By the early 1990s, SRF technology had matured such that complex large-scale systems were credible and the variety of designs and applications began to blossom.

    The TESLA springboard

    In 2020, the TESLA Technology Collaboration (TTC) celebrates 30 years of collaborative efforts on SRF technologies. The TTC grew out of the first international TESLA (TeV Energy Superconducting Linear Accelerator) workshop, which was held at Cornell University (US) in July 1990. Its aim was to define the parameters for a superconducting linear collider for high-energy physics operating in the TeV region and to explore how to increase the gradients and lower the costs of the accelerating structures. It was clear from the beginning that progress would require a large international collaboration, and the Cornell meeting set in motion a series of successes that are ongoing to this day – including FLASH (DE) and the European XFEL at DESY (DE). The collaboration also led to proposals for several large SRF-based research facilities including SNS (US), LCLS-II (US), ESS (SE), PIP-II (US) and SHINE, as well as a growing number of smaller facilities around the world.

    Accelerating gradients above 40 MV/m are now attainable with niobium.

    At the time of the first TESLA collaboration meeting, the state-of-the-art in accelerating gradients for electrons was around 5 MV/m in the operating SRF systems of TRISTAN at KEK (JP), HERA at DESY (DE), LEP-II at CERN (CH) and CEBAF at Jefferson Lab (JLab) (US), which were then under construction. Many participants in this meeting agreed to push for a five-fold increase in the design accelerating gradient to 25 MV/m to meet the dream goal for TESLA at a centre-of-mass energy of 1 TeV. The initial focus of the collaboration was centred on the design, construction and commissioning of a technological demonstrator, the TESLA Test Facility (TTF) at DESY. In 2004, SRF was selected as the basis for an International Linear Collider (ILC) design and, shortly afterwards, the TESLA collaboration was re-formed as the TESLA Technology Collaboration with a scope beyond the original motivation of high-energy physics. The TTC, with its incredible worldwide collaboration spirit, has had a major role in the growth of the SRF community, facilitating numerous important contributions over the past 30 years.

    30 years of gradient march

    Conceptually, the objective of simply providing “nice clean” niobium surfaces on RF structures seems pretty straightforward. Important subtleties begin to emerge, however, as one considers that the high RF-surface currents required to support magnetic fields up to ~100 mT flow only in the top 100 nm of the niobium surface, which must offer routine surface resistances at the nano-ohm level over areas of around 1 m2. Achieving blemish-free, contamination-free surfaces that present excellent crystal lattice structure even in this thin surface layer is far from easy.

    The march of progress in cavity gradient for linacs and the many representative applications over the past 50 years (see figure “Gradient growth”) are due to breakthroughs in three main areas: material purity, fabrication and processing techniques. The TTC had a major impact on each of these areas.

    Gradient growth SRF linac accelerating gradient achievements and application specifications since 1970. CW SRF Linacs – SCA: Stanford Superconducting Accelerator (US); MUSL: lllinois Microtron Using a Superconducting Linac (US); CEBAF: Continuous Electron Beam Accelerator Facility (US); JLab FEL: JLab Free Electron Laser (US); ELBE: HZDR Electron Linear accelerator with high Brillance and Low Emittance (DE); ALICE (CH): STFC Accelerators and Lasers In Combined Experiments (UK); ARIEL: TRIUMF Advanced Rare IsotopE Laboratory (CA); LCLS-II: Linac Coherence Light Source extension (US); SHINE: Shanghai High Brightness Photon Facility (CN). Pulsed SRF Linacs – FAST: Fermilab Accelerator Science and Technology Facility (US); STF: KEK Superconducting RF Test Facility (JP); E-XFEL: European X-Ray Free Electron Laser (DE); ILC: International Linear Collider. Credit: Source: R Geng/JLab (US).

    With some notable exceptions, bulk niobium cavities fabricated from sheet stock material have been the standard, even though the required metallurgical processes present challenges. Cycles of electron-beam vacuum refining, rolling, and intermediate anneals are provided by only a few international vendors. Pushing up the purity of deliverable material required a concerted push, resulting in the avoidance of foreign material inclusions, which can be deadly to performance when uncovered in the final step of surface processing. The figure-of-merit for purity is the ratio of room-temperature to cryogenic normal-conducting resistivity – the residual resistance ratio, RRR. The common cavity-grade niobium material specification has thus come to be known as high-RRR grade.

    Another later pursuit of pure niobium is the so-called “large grain” or “direct-from-ingot” material. Rather than insist on controlled ~30 µm grain-size distribution (grains being microcrystals in the structure), this mat­erial uses sheet slices cut directly from large ingots having much larger, but arbitrarily sized, grains. Although not yet widely used, this material has produced the highest gradient TESLA-style cavities to date – 45 MV/m with a quality factor Q0 > 1010. Here again, though the topic was initiated at JLab, this fruitful work was accomplished via worldwide international collaborations.

    As niobium is a refractory metal that promptly cloaks itself with about 4 nm of dielectric oxide, welding niobium components has to be performed by vacuum electron beam welding. Collaborative efforts in Europe, North America and Asia refined the parameters required to yield consistent niobium welds. The community gradually realised that extreme cleanliness is required in the surface-weld preparation, since even microscopic foreign material will be vaporised during the weld process, leaving behind small voids that become performance-limiting defects.

    Having the best niobium is not sufficient, however. Superconductors have inherent critical magnetic field limitations, or equivalently local surface-current density limitations. Because the current flow is so shallow, local magnetic field enhancements induced by microscopic topography translate into gradient-limiting quench effects. Etching of fabricated surfaces has routinely required a combination of hydrofluoric and nitric acids, buffered with phosphoric acid. This exothermic etching process inherently yields step-edge faceting at grain boundaries, which in turn creates local, even nanoscopic, field enhancements, anomalous losses and quenches as the mean surface field is increased. A progression of international efforts at KEK, DESY, CEA-Saclay and JLab eliminated this problem through the development of electro-polishing techniques. Following a deeper understanding of the underlying electrochemistry, accelerating gradients above 40 MV/m are now attainable with niobium.

    Another vexing problem that TTC member institutions helped to solve was the presence of “Q-drop” in the region of high surface magnetic field, for which present explanations point to subtle migration of near-surface oxygen deeper into the lattice, where it inhibits the subsequent formation of lossy nanohydrides on cool-down. Avoidance of nanohydrides, whose superconductivity by proximity effect breaks down in the Q-drop regime, is required to sustain accelerating gradients above 25 MV/m for some structures.

    Cleaning up

    TTC members have also shared analyses and best practices in cleaning and cleanroom techniques, which have evolved dramatically during the past 30 years. This has helped to beat down the most common challenge for developers and users of SRF accelerating cavities: particulate-induced field emission, whereby very high peak surface electric fields can turn even micron-scale foreign material into parasitic electron field emission sources, with resulting cryogenic and radiation burdens. Extended interior final rinsing with high-pressure ultra-pure water prior to cavity assembly has become standard practice, while preparation and assembly of all beamline vacuum hardware under ISO 4 cleanroom conditions is necessary to maintain these clean surfaces for accelerator operations.

    Vertical test The first cavity of the ESS elliptical section developed by INFN, seen here on the left (next to a large-grain prototype), being prepared for a vertical test at DESY before shipment to CEA Saclay for assembly in the cryomodule. Credit: D Sertore/INFN (IT).

    The most recent transformation has come with the recognition that interstitial doping of the niobium surface with nitrogen can reduce SRF surface resistance much more than was dreamed possible, reducing the cryogenic heat load to be cooled. While still the subject of material research, this new capability was rapidly adopted into the specification for LCLS-II cavities and is also being considered for an ILC. The effort started in the US and quickly propagated internationally via the TTC, for example in cavity tests at the European Spallation Source (SE) (see “Vertical test” image). Earlier this year, Q-values of 3–4 × 1010 at 2 K at 30 MV/m were reported in TESLA-style cavities – representing tremendous progress, but with much optimisation still to be carried out.

    One of the main goals of the TTC has been to bridge the gap between state-of-the-art R&D on laboratory prototypes and actual accelerator components in operating facilities, with the clear long-term objective to enable superconducting technology for a TeV-scale linear collider. This objective demanded a staged approach and intense work on the development of all the many peripherals and subcomponents. The collaboration embraced a joint effort between the initial partners to develop the TTF at DESY, which aimed to demonstrate reliable operation of an electron superconducting linac at gradients above 15 MV/m in “vector sum” control – whereby many cavities are fed by a single high-power RF source to improve cost effectiveness. In 1993 the collaboration finalised a 1.3 GHz cavity design that is still the baseline of large projects like the European XFEL, LCLS-II and SHINE, and nearly all L-band-based facilities.

    Towards a linear collider

    An intense collaborative effort started for the development of all peripheral components, for example power couplers, high-order mode dampers, digital low-level RF systems and cryomodules with unprecedented heat load performances. Several of these components were designed by TTC partners in an open collaborative and competitive effort, and a number of them can be found in existing projects around the world. The tight requirements imposed by the scale of a linear collider required an integrated design of the accelerating modules, containing the cavities and their peripheral components, which led to the concept of the “TESLA style” cryomodules, variants of which provide the building blocks of the linacs in TTF, European XFEL, LCLS-II and SHINE.

    Low-beta cavities A half-wave resonator string assembly at Argonne National Laboratory (US) for use in PIP-II at Fermilab (US). Credit: M Kelly/ANL.

    The success of the TTF, which delivered its first beam in 1997, led it to become the driver for a next-generation light source at DESY, the VUV-FEL, which produced first light in 2005 and which later became the FLASH facility. The European XFEL built on this strong heritage, its large scale demanding a new level of design consolidation and industrialisation. It is remarkable to note that the total number of such TESLA-style cavities installed or to be installed in presently approved accelerators is more than 1800. Were a 250 GeV ILC to go ahead in Japan, approximately 8000 such units would be required. (Note that an alternative proposal for a high-energy linear collider, the Compact Linear Collider, relies on a novel dual-beam acceleration scheme that does not require SRF cavities.)

    Since the partners collaborating on the early TESLA goal of a linear collider were also involved in other national and international projects for a variety of applications and domains, the first decade of the 21st century saw the TTC broaden its reach. For example, we started including reports from other projects, most notably the US Spallation Neutron Source, and gradually opened to the community working on low-beta ion and proton superconducting cavities, such as the half-wave resonator string collaboratively developed at Argonne National Lab and now destined for use in PIP-II at Fermilab (see “Low-beta cavities” image). TTC meetings include topical sessions with industries to discuss how to shorten the path from development to production. Recently, the TTC has also begun to facilitate collaborative exchanges on alternative SRF materials to bulk niobium, such as Nb3Sn and even hybrid multilayer films, for potential accelerator applications.

    Sustaining success

    The mission of the TTC is to advance SRF technology R&D and related accelerator studies across the broad diversity of scientific applications. It is to provide a bridge for open communication and sharing of ideas, development and testing across associated projects. The TTC supports and encourages the free and open exchange of scientific and technical knowledge, engineering designs and equipment. Furthermore, it is based on cooperative work on SRF accelerator technology by research groups at TTC member institution laboratories and test facilities. The current TTC membership consists of 60 laboratories and institutes in 12 countries across Europe, North America and Asia. Since progress in cavity performance and related SRF technologies is so rapid, the major TTC meetings have been frequent.

    Global view Distribution of superconducting particle accelerators using SRF structures for electrons (orange), protons (purple) and heavy ions (pink). More than 30 SRF accelerators are in operation (circles), approximately 15 are presently under construction (triangles) and more than 10 future projects are under consideration (squares). Credit: CERN (CH).

    Particle accelerators using SRF technologies have been applied widely, from small facilities for medical applications up to large-scale projects for particle physics, nuclear physics, neutron sources and free-electron lasers (see “Global view” figure). Five large-scale (> 100 cavities) SRF projects are currently under construction in three regions: ESS in Europe, FRIB and LCLS-II in the US, and SHINE (China) and RAON (Korea) in Asia. Close international collaboration will continue to support progress in these and future projects, including SRF thin-film technology relevant for a possible future circular electron–positron collider. Perhaps the next wave of SRF technology will be the maturation of economical small-scale applications with high multiplicity and international standards. As an ultimate huge future SRF project, realising an ILC will indeed require sustained broad international collaboration.

    The open and free-exchange model that for 30 years has enabled the TTC to make broad progress in SRF technology is a major contribution to science diplomacy efforts on a worldwide scale. We celebrate the many creative and collaborative efforts that have served the international community well via the TESLA Technology Collaboration.

    See the full article here .

    Please help promote STEM in your local schools.

    Stem Education Coalition



    CERN/ATLAS detector


    CERN/ALICE Detector

    CERN CMS New

    CERN LHCb New II


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    SixTRack CERN LHC particles

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